Oil has occupied the premier position as fuel for a long time in contract to the use of coal having the form of a solid. However, since the oil crisis, new merits have been discovered in coal and there have been numerous attempts to handle coal as a fluid by mixing it with a fluid medium, e.g., by a coal oil mixture. However, because a coal oil mixture cannot avoid the defect that it contains oil forming about one-half of its content; development of other slurry fuels has been demanded.
In recent years, attempts to prepare a coal-water fluid slurry suitable for pipeline transportion and for oil tanker transportation by dispersing coal in a water medium in a high concentration, and further to use the slurry in boilers as an alternative fuel for oil, have been carried out. Such slurry involves problems such as an increase in cost due to transportation of water in surplus when transported on ship and a lowering in heating value when burned as a fuel.
To minimize these problems, it is necessary to raise the concentration of coal in the slurry as much as possible, and to do so it becomes essential to add a dispersing agent, such as a surface active agent, to the slurry. However, countermeasures should be taken to make it possible to use as small an amount of surface active agent as possible, because these surface active agents are expensive.
As a method for preparing a coal-water slurry, a method of wet grinding of the coal is more often used than is the method by mixing dry coal powder with water, because of the former method's merits in preventing problems such as the formation of dust, the risk of ignition, and in the direct grinding of coal without drying after coal preparation.
For preparing a coal slurry of high concentration by the wet grinding of coal, a dispersing agent such as a surface active agent has heretofore been added into a grinding mill in advance of any grinding. However, large amounts of surface active agent are required, resulting in economically unattractive increase in the cost of coal-water slurry.